Forty-nine patients with acute optic neuritis (in 21 cases associated with MS) and 26 patients known to have MS but with no history of optic nerve disease underwent visual evoked potential, contrast sensitivity and color vision tests. In patients with optic neuritis the contrast sensitivity was shown to detect optic nerve lesions better than the VEP an often permitted a distinction between acute and past optic neuritis. Combined testing with contrast sensitivity and VEP was superior to the single tests and detected 100% of the acute optic nerve lesions, although in many cases damage was selective and only involved some of the information channels. Desaturated color tests were abnormal in 3/4 of the patients, disturbances of blue-yellow discrimination being commoner than those of red-green. In cases with clinically unilateral optic neuritis the apparently normal partner eye was affected in 61% of the patients; complete recovery of optic nerve function without some residual deficit is rare. Approximately 3/4 of the eyes of patients known to have MS but with no history of past visual disturbances showed bilateral optic nerve involvement. The frequency of subclinical optic nerve lesions rose to 91% at a follow-up examination one year later. the literature is reviewed and our results are compared with the previously published data.